(a) Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display and a manufacturing method thereof.
(b) Description of the Related Art
Recently, the trend for many display applications has been to become thinner and lighter without shrinking the size of the display area. Flat panel displays, which offer characteristics in line with this general trend, are therefore becoming increasingly popular as substitutes for conventional cathode ray tubes (CRT).
Flat panel displays include liquid crystal display (LCD), field emission display (FED), organic light emitting diode (OLED) display, and plasma display panel (PDP), among others. Among the flat panel displays, the OLED display, in many ways, has the most promising future because of its low power consumption, fast response time, wide viewing angle, and high contrast ratio.
An OLED display is a self-emissive display device that includes two electrodes and an organic light emitting layer interposed therebetween. One of the two electrodes injects holes and the other of the two electrodes injects electrons into the light emitting layer. The injected electrons and holes are combined to form excitons, and the excitons emit light as discharge energy upon changing its energy state.
The OLED displays may be divided into passive matrix OLED displays and active matrix OLED displays according to the driving method.
The passive matrix type OLED display includes a plurality of anode lines, a plurality of cathode lines extending perpendicularly to the anode lines, and a plurality of pixels. Each of the pixels includes a light emission layer, and the selection of one of the anode lines and one of the cathode lines causes light emission of the pixel located near where the selected signal overlap. In the active matrix type OLED display, each of a plurality of pixels includes a switching transistor, a driving transistor, and a storage capacitor, as well as an anode, a cathode, and a light emission layer. The driving transistor receives a data voltage from the switching transistor and drives a current having a magnitude that depends on the data voltage. The current from the driving transistor enters the light emission layer, causing emission of a light whose intensity depends on the current.
OLEDs are classified into a bottom-emission type and a top-emission type depending on the light emitting direction. In the bottom-emission type OLED, the primary light-exit surface is located at the bottom of a substrate provided with a thin film transistor. In contrast, the primary light-exit surface is located at the top of the device in the top-emission type OLED.
In the bottom-emission type OLED, light does not transmit through the area that has a thin film transistor and the signal lines, such as wiring. In the top-emission type OLED, on the other hand, the size of the light emission area does not depend on whether the area has the thin film transistor and the wiring. Accordingly, the aperture ratio of the top-emission type may be greater than the aperture ratio of the bottom-emission type.
However, in the top-emission type OLED, there may be a height difference between the regions having the TFTs and the wiring and the other regions, creating a non-uniform thickness in the light emitting layer. This non-uniformity is especially pronounced when the light emitting layer is made of soluble materials.